In certain control applications, a sensor generates an analog signal indicative of the condition being sensed such as fluid level, angular displacement, pressure levels, etc. In order to process the information contained in the signal in modern control systems, it is necessary to convert the analog signal to a digital signal having a certain number of bits of a digital word. The device used is an analog to digital converter, A/D converter.
One input to the A/D converter is, of course the analog signal to be converted. Two other inputs to the A/D converter are for receiving reference voltages which define the range of the input signal. By controlling the reference voltages to the A/D converter, the invention is practiced.
U.S. Pat. No. 4,244,004 issued to Yamada on Jan. 6, 1981, teaches a method of sampling an analog signal to define digitally the signal for use in a picture reproducing machine. Yamada teaches a method wherein digital signals are produced at a certain sampling rate corresponding to an analog signal. The values representative of the analog signal are produced at a frequency which is some multiple "n" of the sampling rate and the digital signal is calculated from "n" consecutively-produced ones of these representative values.
In U.S. Pat. No. 4,558,303 issued to Fielden on Dec. 10, 1985, a method of converting an analog voltage to a digital representation is taught. This is an A/D converter wherein a ramp generator generates a ramping voltage which is summed with the unknown voltage signal and the result is compared with the several outputs of the electronic switching network which is working across the reference voltage range. When there is a comparison, a microprocessor makes the calculations necessary to determine the magnitude of the unknown voltage signal.